ipg: fix jumbo frame compilation

Make jumbo frame support compile again. It was broken by the cleanup series
before the merge because the code is hidden under JUMBO_FRAME #ifdef.

Tested-by: Andrew Savchenko <Bircoph@list.ru>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>